This invention relates to an apparatus for measuring physical properties of golf club shafts, specifically to an apparatus for measuring the torsional stiffness (torsional spring constant) of a golf club shaft.
In the field of designing, modifying and fitting golf clubs, it is advantageous to know the physical properties of the golf club shaft as well as the physical properties of the golf club head. It is common in the industry to rate clubs based on the flexural stiffness designated typically by the terms: Extra Stiff (XS); Stiff (S); Firm (F); Regular (R); Average (A); and Ladies"" (L). The flexural stiffness is important, of course, because it determines the maximum bending as well as the first bending mode frequency of the shaft and, therefore, by selecting the appropriate shaft stiffness, the club can be optimized for the swing speed of the particular golfer. The torsional stiffness of the golf club shaft is of equal importance because it determines the maximum windup of the club head relative to the shaft and the torsional frequency at which the club head oscillates about the axis of the golf club shaft during the swing. For optimum performance, in addition to matching the flexural stiffness of the shaft to the player""s swing speed, the torsional stiffness of the shaft should also be matched to the club head swing weight and the player""s swing speed.
Prior art methods for determining the torsional stiffness of a golf club shaft comprise fixing one end of the shaft in a collet or chuck then applying a predetermined torque (one to five foot pounds is a standard) to the opposite end of the shaft. The total angular deflection of the shaft is then read. In order to obtain sufficient resolution, a relatively high torque, must be used, therefore, in order to ensure the shaft does not move in the fixture under the high torque, a relatively high clamping force must be used. This results in many of the exotic composite shafts being crushed by the clamping force exerted by the collet on the end of the shaft. In order to avoid crushing the composite shafts, many manufacturers have resorted to using a lower torque on composite shafts and high torque on steel shafts, with the result that there is now no longer a consistent scale to compare the flexural stiffness of composite shafts to steel shafts or indeed in many cases clubs of the same type from manufacturer to manufacturer. Accordingly, what is needed is an apparatus for measuring the torsional stiffness of a golf club shaft that has sufficient resolution to measure small differences in the torsional stiffness of steel shafts without requiring high torque (and high clamping forces) that can damage composite shafts.
The present invention satisfies the foregoing need by providing an apparatus and method for determining torsional stiffness of a shaft by measuring the torsional frequency at which the shaft oscillates when coupled to a torsional weight having a known mass moment of inertia. In a preferred embodiment of an apparatus for determining the torsional stiffness of a golf club shaft incorporating features of the present invention, a collet is provided for securing one end of a shaft to the rigid frame of the apparatus. A second collet secures the opposite end of the shaft to an inertial weight. The inertial weight has mounted to it a conventional biaxial accelerometer. When the inertial weight is displaced from the initial static position and released, the inertial weight oscillates about its center of mass under the urging of the torsional stiffness of the golf club shaft. The accelerometer provides a signal indicative of the amplitude and frequency of oscillation of the inertial mass which is fed into a computer. The computer is programmed with the mass moment of inertia of the inertial weight and therefore is able to solve the differential equation of motion for the torsional spring constant of the golf club shaft based on the frequency of oscillation. The torsional spring constant can be displayed directly in Newton meters per radian (or any other engineering unit) or can be converted into units directly comparable to the prior art measurement systems by multiplying the spring constant by the appropriate load (e.g. five foot pounds).